A large number of applications exist in which it is desirable to activate or enable an electronic or computer circuit when a particular object, e.g., a vehicle, undergoes motion. For example, U.S. patent application Ser. No. 524,325 for an invention entitled "Electrical Power Distribution System" assigned to the same assignee as the present invention, discloses an automotive electrical system which can be enabled by a motion sensor. More particularly, as described in the U.S. Patent application referenced above, a preselected portion of a car battery is activated to crank the engine of the car only upon motion of the car, such as what occurs when a person sits down in the driver's seat of the car, to conserve the stored energy in the preselected battery portion.
While the above-referenced U.S. Patent application discloses one application for a motion sensor, it will be appreciated that many more applications exist in which it is desirable to sense the motion of an object and to produce an electrical signal which is representative of the motion. Regardless of the particular application of the motion sensor, it is ordinarily preferred that the motion sensor be relatively inexpensive, rugged, and reliable. It is further preferred and indeed often required that the motion sensor transduce detected mechanical motion into an electrical signal that can advantageously be used in an electrical circuit.
One type of inexpensive, comparatively rugged sensor that can transduce mechanical vibration of the sensor into an electrical signal output is a piezoelectric crystal. More particularly, certain materials, known as piezoelectric materials, generate an electrical voltage signal when the atoms of the crystalline structure of the material which make up the material are disturbed.
The disturbance of the atoms can be caused by, among other things, a pressure wave impinging upon the material. To sense pressure waves of relatively small amplitudes, it is necessary that the pressure wave have a frequency that is at or near the resonant frequency of vibration of the particular piezoelectric structure. It happens that audible sound waves typically exhibit frequencies that are at or near the resonant frequency of vibration of many piezoelectric structures.
As the frequency of a source of an atomic disturbance having a relatively small amplitude decreases to a value substantially below the resonant frequency of vibration of the piezoelectric structure, however, the amplitude of the output electrical signal generated by the structure may decrease to a value which is insufficient to support processing of the signal. Such a low frequency, small amplitude disturbance can be caused by, for example, motion of a vehicle in which the piezoelectric structure is mounted. The present invention recognizes that relatively small, low frequency disturbances of a piezoelectric structure can nevertheless be detected by a motion sensor which operates on the principle of piezoelectricity.
Accordingly, it is an object of the present invention to provide a piezoelectric motion sensor which can sense motion having a comparatively low frequency. It is another object of the present invention to provide a piezoelectric motion sensor that can be used to detect motion of a vehicle. A further object of the present invention is to provide a piezoelectric motion sensor that is relatively easy to use and comparatively cost-effective to manufacture.